Electrical Alternator???

Discussion in 'Fox 5.0 Mustang Tech' started by RangerJoe, Jul 17, 2013.

  1. My DD, Ranger, blew a head gasket and I have been driving the Stang as a DD the last two weeks. Lots of rain here in GA recently, and the Stang is parked outside while the Ranger is being put back together. Two seperate, nonconsecutive days now, I have come outside and found my battery light on, while the car has been off. It goes away while the car is running, but then comes back on.

    I have been searching old post, most of them point to a bad regulator on the alternator, bad alternator or bad ignition switch. I replaced my ignition switch three months ago, for unrelated reasons. Alternator is 2 years old, bought brand new, and is a 3G. Haven't had time to really trouble shoot between work, the kid, and trying to get my Ranger back together.

    Can anyone vouch for either of these being the culprit, or have any other suggestions? I mentioned the rain because I wondered if too much moisture could harm the regulator.

    Thanks in advance.

  2. You either have a drain somewhere in your electrical system or a bad diode in the alternator.

    Either could be diagnosed using a DVM to see where the current draw is going when the car is off.
  3. sounds like a trip to autozone for a free check is in your future.
  4. If you pull the 18 fuse with the car off does the light go out? I've seen that before several years back and turned out the alternator was bad.
  5. Never seem to have luck with AZ testing things. I do buy alot from them though. The Alternator came from a private shop, I will check with him for testing.

  6. Thanks for the suggestion. I will give this a try when I get off work. Bad alternator, or bad diode?

    Thanks again.

  7. Alternator troubleshooting for 86-93 5.0 Mustangs:

    Never, never disconnect an alternator from the battery with the engine running. The resulting voltage spike can damage the car's electronics including the alternator.

    Revised 15 April 2012 to add simple check for regulator failure in Engine off ignition on, battery fully charged section, item 2.

    Red color text applies to cars with a 3G alternator.

    Do all of these tests in sequence. Do not skip around. The results of each test depend on the results of the previous tests for correct interpretation.

    Simple first step: Remove the alternator and take it to your local auto parts store. They can bench test it for free.

    Engine off, ignition off, battery fully charged.
    1.) Look for 12 volts at the alternator output. No 12 volts and the dark green fuse link between the orange/black wires and the battery side of the starter solenoid has open circuited.
    3G alternator: Look for 12 volts at the stud on the back of the alternator where the 4 gauge power feed wire is bolted.
    No voltage and the fuse for the 4 gauge power feed wire is open or there are some loose connections.

    2.) Look for 12 volts on the yellow/white wire that is the power feed to the regulator. No 12 volts, and the fuse link for the yellow/white wire has open circuited.

    Engine off, ignition on, battery fully charged:
    1.) Alternator warning light should glow. No glow, bulb has burned out or there is a break in the wiring between the regulator plug and the dash. The warning light supplies an exciter voltage that tells the regulator to turn on. There is a 500 ohm resistor in parallel with the warning light so that if the bulb burns out, the regulator still gets the exciter voltage.
    Disconnect the D connector with the 3 wires (yellow/white, white/black and green/red) from the voltage regulator.
    Measure the voltage on the Lt green/red wire. It should be 12 volts. No 12 volts and the wire is broken, or the 500 ohm resistor and dash indicator lamp are bad. If the 12 volts is missing, replace the warning lamp. If after replacing the warning lamp, the test fails again, the wiring between the warning lamp and the alternator is faulty. The warning lamp circuit is part of the instrument panel and contains some connectors that may cause problems.

    2.) Reconnect the D plug to the alternator
    Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see 2.4-2.6 volts. No voltage and the previous tests passed, you have a failed voltage regulator. This is an actual measurement taken from a car with a working electrical system. If you see full or almost full12 volts, the regulator has failed.

    Engine on, Ignition on, battery fully charged:
    Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see battery voltage minus .25 to 1.0 volt. If the battery measured across the battery is 15.25 volts, you should see 14.50 volts

    Familiarize yourself with the following application note from Fluke: See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .

    You will need to do some voltage drop testing of several of the wires.

    Start looking for these things:
    1.) Bad diode(s) in the alternator - one or more diodes have open circuited and are causing the voltage to drop off as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.

    2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Do the voltage drop test as shown in the Fluke tech note link. Measure the voltage drop between the alternator frame and the battery negative post. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.

    3.) Bad regulator that does not increase field current as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.

    4.) Bad sense wire - open circuit in sense wiring or high resistance. The yellow/white wire is the voltage sense and power for the field. There is a fuse link embedded in the wiring where it connects to the black/orange wiring that can open up and cause problems. Disconnect the battery negative cable from the battery: this will keep you from making sparks when you do the next step. Then disconnect the yellow/white wire at the alternator and the green fuse link at the starter solenoid/starter relay. Measure the resistance between the alternator end of the yellow/white wire and the green fuse link: you should see less than 1 ohm. Reconnect all the wires when you have completed this step.

    5.) Bad power feed wiring from the alternator. Use caution in the next step, since you will need to do it with everything powered up and the engine running. You are going to do the Fluke voltage drop tests on the power feed wiring, fuse links and associated parts. Connect one DMM lead to the battery side of the starter solenoid/starter relay. Carefully probe the backside of the black/orange wire connector where it plugs into the alternator. With the engine off, you should see very little voltage. Start the engine and increase the load on the electrical system. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.


    Alternator wiring circuit
    Notice the green wire connects to a switched power source. The circuit contains a 500 ohm resistor in series between the switched power and the alternator. Connecting it to switched power keeps the regulator from drawing current when the engine is not running. The resistor limits the current flowing through the wire so that a fuse isn't needed if the wire shorts to ground.

    Also notice the sense wire connects to the starter solenoid and it is fused. It connects to the starter solenoid so that it can "sense" the voltage drop across the output wiring from the alternator.
  8. Thanks J. Doing the final flush on my Ranger after head gasket repair. Have to work this weekend, but this will be my first thing to do Monday.

  9. Alright Joe, here is where I'm at. While waiting on the Ranger's flush to cool, I printed out the check list. I skipped step 1, which is bench testing. So sorry...(really, I'm stuck at home right now)

    Got to the part where IGNITION IS ON, ENGINE OFF:

    My battery light glows when ignition is in the off position, but as soon as you turn the ignition on, the battery light goes off. No biggie (I hope). I disconnected the D Plug, got 12.22 volts on the green/red wire. I reconnected the D Plug, and measured green/red wire from the back side of connector, and got 11.20 volts.

    So, I conclude from your list that I need a voltage regulator. I will have alternator checked tomorrow, but if the alternator itself checks out okay, will it be okay to just purchase the regulator?

    Thanks again for your help!

  10. The regulator is built into the alternator and is about $30 at AutoZone, the rebuilt alternator is about $80 . It's a matter of what your wallet and time can afford. Before replacing the regulator, disassemble the alternator and check the slip rings and the brushes to make sure they aren't worn out. Getting the brushes back it may be a bit of a challenge. Usually you put them in place and then use a straightened paper clip to hold them in place when you reassemble the alternator. I can't be sure that is the way that the stock alternator works, but the odds are that's the way it is done.
  11. Alright J, found my problem, it was not the regulator. Haven't made it to the parts store yet, but will probably get it bench tested just for giggles. What clued me into this was the fact that the battery light stayed on, even after I removed the alternator from my car all together. After studying the wire diagram, it became clear that with the regulator removed from car, the light should go out.

    So, after several minutes trouble shooting, I narrowed it down to my Taylor battery cutoff switch. I used your diagram when I installed it, which called for running the green/red wire to the switch to kill the alternator. I bypassed the switch, and presto, no more battery light. The switch must have internally shorted out or something, and was allowing power from the main battery cables to trickle into the green/red wiring. Perhaps the Taylor is not all that great for rainy conditions.

    On a second note, I also found out that my stock (2) 10 gauge alternator power wires have burnt fusible links. They are only measuring .26 volts each, while my additional 4 gauge wire is carrying a full current. I have seen some people delete these (2) 10 gauge wires all together with the 3G upgrade. Are they necessary, or do I need to replace the fusible links? Obviously I need to trouble shoot and make sure they are not shorted somewhere.

    Thanks again for all your help!

  12. Just went back out and tried a continuity test to ground with the (2) 10 gauge alternator wires. They are not shorted to ground. I have no idea how long they have been bad. Since I have been DD the Stang for the last two weeks, I have been running the AC wide open, which in turn triggers my dual Contour fans (40+ amps), along with my MSD and other components. Would this large of a load cause the fusible links to burn? My 4 gauge wire has a 150 amp fuse, and apparently has been carrying the load.

    Thanks again!

  13. Unless the Taylor switch has the same extra switch contacts (Double Pole, Single Throw = DPST) as the Moroso switch I recommended, it won't work correctly. The extra set of contacts allow you to separate the battery from the alternator exciter circuit (Dash Warning light and 500 ohm resistor circuit). Wiring them all on the same switch contacts leaves the alternator exciter powered on anytime the main battery switch is turned on. That may be the source of your stray battery drain.

    All resistance measurements on the alternator circuit and wiring need to be made with the negative terminal of the battery disconnected from the wiring. If you let it connected, the small current that the radio presets and computer keep alive memory circuits draw can throw off your resistance checks.

    The 10 gauge fuse links in the black/orange wires don't serve much purpose once the fused 4 gauge wire is installed to carry the main alternator output. The main thing to be concerned about is that the yellow/white wire for the alternator sense and field power is connected to the same fuse link that the black/orange wires use. If that fuse link opens up (blows) then the alternator has no field power, and as a result, no output.
  14. Hmmmmm, interesting concerning the yellow white wire, because I got power out of it on my original testing. I'm starting to wonder if I let the 10 gauge wires ground themselves during my testing. Time to go back and check that wire again.

    Yes, the Taylor switch is just like the moroso, 4 prong. It has worked well now for almost 4 months. It has internally shorted somehow, i think from all the rain I have been driving it in. I've got it bypassed now, will probably order a moroso in the nears future.

    Thanks again, if the yellow/white wire has no power, then I will definetly have to fix.

  15. Tested the white/yellow wire on my way out the door tonight. Looks like i blew the fuse link for the 10 gauge wires. Last question for you, it there just one fuse link for both 10 gauge wires, or does it use two? If just one, what size do i need? I would think larger than 10 gauge. My wires are tucked, so I can't look at them right now.

    Thanks again for all your help!
  16. One fuse link for both black/orange wires.
  17. Awesome, just picked one up. Thanks again!